182 SECTIONAL ADDRESSES. 



carried in tlie sex-chromosomes, a clean separation of homologous 

 members at meiosis shotild result in the characters which were asso- 

 ciated m the parents remaining strictly in the same combination in 

 each succeeding generation. The fact that this is not the case has 

 led Morgan to conclude that an interchange of qhromosome material 

 must take place at this phase among a proportion of the gametes, and 

 that the percentage of these ' cross-overs ' will depend on the distance 

 apart of the loci of the factors concerned. This phenomenon of linkage 

 may also be exhibited by pairs of characters wliich show no sex- 

 linkage in their inheritance. The factors involved in these latter cases 

 must presumably, therefore, be disposed in one of the chromosomes 

 which is not the sex-chromosome. 



To this brief sketch of the main points of Morgan's chromosome 

 theory must be added mention of the extremely interesting 

 relation which lends strong support to his view, and the significance of 

 which seems scarcely to admit of question, viz. : that in Drosophila 

 avjpelophila there are four pairs of chromosomes, and that the linkage 

 relations of the hundred and moi'e characters investigated indicate that 

 they form four distinct groups. It is hardly possible to suppose that 

 the one fact is not directly connected with the other. The interesting 

 discovery of Bridges ^^ that the appearance of certain unexpected cate- 

 gories among Drosophila offspring, where females of a particular 

 strain were used, coincided with the presence in these females of an 

 additional chromosome adds another link in the chain of evidence. On 

 examination it was found that in these females the X chromosome pair 

 occasionally failed to separate at the reduction division, and conse- 

 quently that the two XX chromosomes sometimes both remained in the 

 egg, and sometimes both passed out into the polar body. Hence there 

 arose from fertilisation of the XX eggs some individuals containing 

 three sex-chromosomes, with the resulting upset of the expectation in 

 regard to sex-limitation of characters which was observed. 



It, however, remains a curious anomaly that in the cross-bred 

 Drosophila male no corresponding crossing-over of linked characters, 

 whether associated with the sex character or not, has yet been ob- 

 served. His gametes carry only the same factorial combinations 

 which he received from his parents. For this contrast in the behaviour 

 between the sexes there is at present no explanation. The reverse con- 

 dition has been described by Tanaka" in the silkworm. Here inter- 

 change takes place in the male but not in the female. 



It must then be acknowledged that Morgan's interpretation of the 

 cytological evidence has much in its favour. The striking parallel 

 between the behaviour of the chromosomes and the distributional rela- 

 tions of Mendelian allelomorphs is obvious. The existence in Droso- 

 phila ampelophila of four pairs of chromosomes and of four sets of 

 linked characters can hardly be mere coincidence. The employment of 

 the smaller physical unit in accounting for the reshuffling of characters 

 in their transmission commends itself in principle. The necessity for 

 postulating the occurrence of some orderly irregularity in the hereditary 



13 .7. Exp. Zool. XV.. 1913. 



14 /. Coll. Agr., Sapporo, Japan, 191.3-14. 



